data defines what data libcurl wants to unlock, and you must make sure
that only one lock is given at any time for each kind of data.

userptr is the pointer you set with CURLSHOPT_USERDATA.

CURLSHOPT_SHARE

The parameter specifies a type of data that should be shared. This may
be set to one of the values described below.

CURL_LOCK_DATA_COOKIE

Cookie data will be shared across the easy handles using this shared object.

CURL_LOCK_DATA_DNS

Cached DNS hosts will be shared across the easy handles using this shared
object. Note that when you use the multi interface, all easy handles added to
the same multi handle will share DNS cache by default without this having to
be used!

CURLSHOPT_UNSHARE

This option does the opposite of CURLSHOPT_SHARE. It specifies that
the specified parameter will no longer be shared. Valid values are
the same as those for CURLSHOPT_SHARE.

CURLSHOPT_USERDATA

The parameter allows you to specify a pointer to data that will be passed
to the lock_function and unlock_function each time it is called.

RETURN VALUE

CURLSHE_OK (zero) means that the option was set properly, non-zero means an
error occurred as <curl/curl.h> defines. See the libcurl-errors.3
man page for the full list with descriptions.